1.5. Thesis Organization

Finally, the increased concentrations of TRU nuclides in the fuel cycle would complicate the fuel handling, reprocessing, and fabrication due to increase levels of radiation. Some of the higher MA nuclides mentioned above in particular Cm244, Cm246, Cm248, and Cf252 have relatively high branching ratio to spontaneous fission (SF). The neutrons released in SF process are very hard to shield so that accumulation of SF neutrons emitting nuclides would aggravate the TRU containing fuel handling and fabrication problem. Additionally, the TRU separation based on the aqueous reprocessing techniques may not be suitable for the reprocessing of fuels with high TRU concentrations. The organic solvents used for separation of TRU from the fission products degrade rapidly when exposed to high radiation or elevated temperatures caused by the decay of TRU. As a result, non-aqueous separation techniques based on pyrochemical or pyrometallurgical reprocessing may be required.

This thesis consists of ten chapters. The first chapter briefly introduced the problematic issues associated with once-through fuel cycle such as accumulation of long lived radioactive waste, proliferation resistance, and poor utilization of natural resources. Then, short overview of the suggested in the past solutions and strategies was presented. The possibility of addressing the most important once through fuel cycle issues using exclusively the existing LWRs was proposed and justified. The list of potential associated technical challenges was subsequently presented.

The evaluation of neutronic performance of the two most promising cycles, with respect to the TRU destruction capabilities, fuel matrix candidates is presented in Chapters 2 and 3 focusing on thorium oxide and fertile free fuel matrices respectively. TRU destruction rates and TRU residual fractions in the spent fuel are the main parameters of interest in the discussion. The reactivity feedback coefficients and soluble boron reactivity worth are also reported as primary indicators of practical fuel design feasibility.

Chapter 4 compares the evaluated thorium oxide and fertile free fuel options in terms of their repository performance characteristics in a once through burndown scenario aiming at the reduction of existing TRU stockpile and at constraining future TRU generation. The performance

of reference once-trough fuel cycle and mixed oxide (MOX) based fuel is also reported for the comparative assessment.

Since fertile free fuel showed superior performance in terms of TRU burning capabilities, it was chosen as the best option for the evaluation of sustainable fuel cycle with zero net production of TRU. In Chapter 5, a heterogeneous Combined Non-fertile and UO2 (CONFU) fuel assembly concept is introduced. The possibility of achieving an equilibrium state with respect to TRU inventory and fuel cycle length is demonstrated.

Chapter 6 addresses feasibility issues of practical CONFU assembly based core design. 3dimensional whole core neutronic simulation results are reported for a number of considered design alternatives including micro- and macro-heterogeneous CONFU assembly design options, fully fertile free TRU containing core, and the reference PWR UO2 fueled core. The analysis is focused on the evaluation of the core power peaking factors, cycle length, reactivity coefficients, and control rods and soluble boron reactivity worth. The performed thermal hydraulic analysis aiming at the evaluation of MDNBR margin for analyzed cases is also reported.

The dynamic behavior of the cores containing TRU in response to the reactivity initiated accidents is analyzed in Chapter 7. A simple computer model created for such an assessment is described and the results of the simulations are presented. Additional concerns related to loss of coolant accident (LOCA) in TRU containing cores are discussed in some details.

Chapter 8 presents the results of environmental hazard characteristics of the sustainable LWR fuel cycle. Some practicality issues related to possibility of the fuel cycle infrastructure facilities to handle multi-recycling of TRU are discussed. The decay heat, photon, and neutron doses from spontaneous fission of TRU are reported as a function of the number of TRU recycle paths. A brief review of the existing and prospective TRU reprocessing options is also presented.

In Chapter 9, the results of the fuel cycle cost and the total cost of electricity estimations for the sustainable LWR based fuel cycle are compared with those of the once-through reference cycle and with alternative (LWR and fast actinide burner reactor) double strata fuel cycle.

Finally, Chapter 10 summarizes the main findings and conclusions of this thesis.

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